1. Field of the Invention
The present invention relates to a magnetic recording medium, particularly, a magnetic recording medium having excellent durability and running property.
2. Description of the Related Art
Since aromatic polyamide films have excellent heat resistance and mechanical properties, they have been studied in various applications. As para-oriented aromatic polyamides have mechanical properties, such as stiffness and strength, which are better than those of other polymers, it has been proposed to use them in printer ribbons, magnetic tapes, capacitors and the like. Para-oriented aromatic polyamides are drawing particular attention as materials appropriate for base films of small video tapes and tapes for external memory of computers. In these applications, for example, video tapes are being used more often outdoors, so that magnetic tapes having excellent durability, which can be safely used even if the tapes encounter large fluctuations of temperature, are demanded. As for tapes used in external memory of computers, since they are often used under severe conditions, such as high temperatures and repeated runnings, and since reliability thereof is strongly demanded, a magnetic tape having excellent running property and durability is demanded.
An example of a known magnetic recording medium employing an aromatic polyamide film is described in Japanese Laid-Open Patent Application (Kokai) No. 58-168655, in which a ferromagnetic layer is formed on an aromatic polyamide film that contains not less than 50 mol % of para-aromatic amide units and has a tensile Young's modulus in at least one direction of not less than 500 kg/mm.sup.2 in order to promote dimensional stability against external force, fluctuation of temperature and humidity. Japanese Laid-Open Patent Application (Kokai) No. 62-112218 discloses a magnetic tape in which a magnetic layer is formed on an aromatic polyamide film having a thickness of 3 .mu.m to 8 .mu.m, a tensile Young's modulus in the longitudinal direction of not less than 1300 kg/mm.sup.2, an edge tearing strength in all directions of not less than 1 kgf, and a light transmittance at 600 nm of not less than 60% in order to reduce jitter, foldings and wrinkles of the tape. U.S. Pat. No. 4,645,702 discloses a magnetic recording medium in which a magnetic layer is formed on an aromatic polyamide film having a density of 1.400 to 1.490 (g/cm.sup.3) and a product of the coefficient of thermal expansion and heat shrinkage factor of 1.0.times.10.sup.-4 to 1.0.times.10.sup.-7 (mm/mm/.degree.C.) * (%), in order to promote flatness after forming the magnetic layer. Japanese Patent Publication (Kokoku) No. 55-9425 discloses a process for producing an aromatic polyamide film having substituent groups introduced into the aromatic polyamide, having an inherent viscosity of not less than 0.55, and having a content of ionic inorganic compounds of not more than 1000 ppm, for the purpose of reducing curl and promoting the heat resistance of the aromatic polyamide film. The ionic inorganic compounds herein mean compounds derived from lithium salts, calcium salts or the like employed as a solubilizer, and sulfuric acid or the like used as a solvent.
However, since magnetic recording media have recently acquired large capacities, are small and thin, and are more and more often used under severe conditions such as high temperatures, a magnetic recording medium having improved running property and durability over that obtained by the above-described prior art techniques is demanded. It is believed the demand will increase even further.
The present inventors looked for factors which control running property and durability, and discovered that running property and durability can be promoted by controlling the oozing of low molecular-weight impurities such as oligomers contained in the film, thereby achieving the present invention.